1. Field of the Invention
The present invention relates to radio communication apparatuses such as mobile telephones, portable telephones, and cordless telephones and, more particularly, to a dual mode radio communication apparatus used in a mobile radio communication system for transmitting a radio communication signal between a base station and a mobile unit by selectively using an analog or digital mode.
2. Description of the Related Art
In recent years, a system using a dual mode scheme has been proposed in place of a mobile radio communication system for transmitting a radio communication signal in accordance with a conventional analog scheme. The dual mode is a scheme for selectively using an analog or digital mode.
In the analog mode, a transmitter transmits a carrier wave modulated with an analog speech signal and data in accordance with, e.g., FM modulation. A receiver receives the modulated wave from the transmitter and FM-demodulates it to reproduce the analog speech signal and data. On the other hand, in the digital mode, a transmitter encodes a speech signal and data, and a carrier wave is digitally modulated with the encoded signals in accordance with, e.g., a .pi./4 shifted DQPSK (.pi./4 shifted, differentially encoded quadrature phase shift keying) scheme. The digitally modulated wave is transmitted from the transmitter. A receiver receives the modulated wave sent from the transmitter and demodulates it, thereby reproducing the speech signal and data.
In recent years, a proposal has been made to cause a mobile unit to selectively use the analog or digital mode in the system described above. According to this scheme, a function switch for designating a mode is arranged in the mobile unit. When a desired mode is designated with this switch, this desired mode is informed to a base station at the time of radio speech channel connection for starting speech communication. The base station searches a non-busy radio speech channel in accordance with the informed desired mode. If any non-busy radio speech channel is found, this radio speech channel is informed to the mobile unit. The base station and the mobile unit are connected via this radio speech channel. Even during speech communication, the operation of the function switch is monitored in the mobile unit. When this function switch is operated, a mode change request is informed to the base station. The base station searches a non-busy radio speech channel in accordance with this request. If a radio speech channel corresponding to the mode desired by the mobile unit is found during this search operation, this radio speech channel is informed from the base station to the mobile unit. As a result, the current radio speech channel is switched to the new radio speech channel. In this system, it is possible to directly transit the state from the state in which normal speech communication is performed using an analog speech channel to the state in which privacy speech communication using a digital speech channel is performed.
In the system of this type, however, various problems are left unsolved in operations for designating a mode in the mobile unit.